John Stuart Mill was born on Rodney Street in the Pentonville area of London, the eldest son of the Scottish philosopher and historian James Mill and Harriet Burrow. John Stuart was educated by his father, with the advice and assistance of Jeremy Bentham and Francis Place. He was given an extremely rigorous upbringing, and was deliberately shielded from association with children his own age other than his siblings. His father, a follower of Bentham and an adherent of associationism, had as his explicit aim to create a genius intellect that would carry on the cause of utilitarianism and its implementation after he and Bentham had died. Mill was a notably precocious child; at the age of three he was taught Greek. By the age of eight he had read Aesop's Fables, Xenophon's Anabasis, and the whole of Herodotus, and was acquainted with Lucian, Diogenes Laërtius, Isocrates and six dialogues of Plato. He had also read a great deal of history in English and had been taught arithmetic. At the age of eight he began learning Latin, Euclid, and algebra, and was appointed schoolmaster to the younger children of the family. His main reading was still history, but he went through all the commonly taught Latin and Greek authors and by the age of ten could read Plato and Demosthenes with ease. His father also thought that it was important for Mill to study and compose poetry. One of Mill's earliest poetry compositions was a continuation of the Iliad. In his spare time, he also enjoyed reading about natural sciences and popular novels, such as Don Quixote and Robinson Crusoe. His father's History of India was published in 1818; immediately thereafter, about the age of twelve, Mill began a thorough study of the scholastic logic, at the same time reading Aristotle's logical treatises in the original language. In the following year he was introduced to political economy and studied Adam Smith and David Ricardo with his father, ultimately completing their classical economic view of factors of production. Mill's comptes rendus of his daily economy lessons helped his father in writing Elements of Political Economy, which became the leading textbook exposition of doctrinaire Ricardian economics. Ricardo, who was a close friend of his father, used to invite the young Mill to his house for a walk in order to talk about political economy.

At age fourteen, Mill stayed a year in France with the family of Sir Samuel Bentham, brother of Jeremy Bentham. The mountain scenery he saw led to a lifelong taste for mountain landscapes. The lively and friendly way of life of the French also left a deep impression on him. In Montpellier, he attended the winter courses on chemistry, zoology, logic of the Faculté des Sciences, as well as taking a course of the higher mathematics. While coming and going from France, he stayed in Paris for a few days in the house of the renowned economist Jean-Baptiste Say, a friend of Mill's father. There he met many leaders of the Liberal party, as well as other notable Parisians, including Henri Saint-Simon.

This intensive study however had injurious effects on Mill's mental health, and state of mind. At the age of twenty he suffered a nervous breakdown. As explained in chapter V of his Autobiography, this was caused by the great physical and mental arduousness of his studies which had suppressed any feelings he might have developed normally in childhood. Nevertheless, this depression eventually began to dissipate, as he began to find solace in the Mémoires of Jean-François Marmontel and the poetry of William Wordsworth.

Mill refused to study at Oxford University or Cambridge University, because he refused to take Anglican orders from the "white devil". Instead he followed his father to work for the East India Company until 1858. In 1851, Mill married Harriet Taylor after 21 years of an intimate friendship. Taylor was married when they met, and their relationship was close but generally believed to be chaste during the years before her first husband died. Brilliant in her own right, Taylor was a significant influence on Mill's work and ideas during both friendship and marriage. His relationship with Harriet Taylor reinforced Mill's advocacy of women's rights. He cites her influence in his final revision of On Liberty, which was published shortly after her death, and she appears to be obliquely referenced in The Subjection of Women. Taylor died in 1858 after developing severe lung congestion, only seven years into their marriage.

Between the years 1865-1868 Mill served as Lord Rector of the University of St. Andrews. During the same period, 1865-8, he was a Member of Parliament for City and Westminster, and was often associated with the Liberal Party. During his time as an MP, Mill advocated easing the burdens on Ireland, and in 1869 became the first person in Parliament to call for women to be given the right to vote. Mill became a strong advocate of women's rights and such social reforms as labor unions and farm cooperatives. In Considerations on Representative Government, Mill called for various reforms of Parliament and voting, especially proportional representation, the Single Transferable Vote, and the extension of suffrage. He was godfather to Bertrand Russell.

He died in Avignon, France, in 1873, where he is buried alongside his wife.

Mill's On Liberty addresses the nature and limits of the power that can be legitimately exercised by society over the individual. One argument that Mill develops further than any previous philosopher is the harm principle. The harm principle holds that each individual has the right to act as he wants, so long as these actions do not harm others. If the action is self-regarding, that is, if it only directly affects the person undertaking the action, then society has no right to intervene, even if it feels the actor is harming himself. He does argue, however, that individuals are prevented from doing lasting, serious harm to themselves or their property by the harm principle. Because no-one exists in isolation, harm done to oneself also harms others, and destroying property deprives the community as well as oneself. Mill excuses those who are "incapable of self-government" from this principle, such as young children or those living in "backward states of society".

Mill argues that despotism is an acceptable form of government for those societies that are "backward", as long as the despot has the best interests of the people at heart, because of the barriers to spontaneous progress. Though this principle seems clear, there are a number of complications. For example, Mill explicitly states that "harms" may include acts of omission as well as acts of commission. Thus, failing to rescue a drowning child counts as a harmful act, as does failing to pay taxes, or failing to appear as a witness in court. All such harmful omissions may be regulated, according to Mill. By contrast, it does not count as harming someone if — without force or fraud — the affected individual consents to assume the risk: thus one may permissibly offer unsafe employment to others, provided there is no deception involved. (Mill does, however, recognize one limit to consent: society should not permit people to sell themselves into slavery). In these and other cases, it is important to keep in mind that the arguments in On Liberty are grounded on the principle of Utility, and not on appeals to natural rights.

On Liberty involves an impassioned defense of free speech. Mill argues that free discourse is a necessary condition for intellectual and social progress. We can never be sure, he contends, that a silenced opinion does not contain some element of the truth. He also argues that allowing people to air false opinions is productive for two reasons. First, individuals are more likely to abandon erroneous beliefs if they are engaged in an open exchange of ideas. Second, by forcing other individuals to re-examine and re-affirm their beliefs in the process of debate, these beliefs are kept from declining into mere dogma. It is not enough for Mill that one simply has an unexamined belief that happens to be true; one must understand why the belief in question is the true one.

Mill believed that “the struggle between Liberty and Authority is the most conspicuous feature in the portions of history.” For him, liberty in antiquity was a “contest... between subjects, or some classes of subjects, and the government." Mill defined "social liberty" as protection from "the tyranny of political rulers." He introduces a number of different tyrannies, including social tyranny, and also the tyranny of the majority. Social liberty for Mill was to put limits on the ruler’s power so that he would not be able to use his power on his own wishes and make every kind of decision which could harm society; in other words, people should have the right to have a say in the government’s decisions. He said that social liberty was “the nature and limits of the power which can be legitimately exercised by society over the individual”. It was attempted in two ways: first, by obtaining recognition of certain immunities, called political liberties or rights; second, by establishment of a system of "constitutional checks". However, limiting the power of government is not enough. "Society can and does execute its own mandates: and if it issues wrong mandates instead of right, or any mandates at all in things with which it ought not to meddle, it practices a social tyranny more formidable than many kinds of political oppression, since, though not usually upheld by such extreme penalties, it leaves fewer means of escape, penetrating much more deeply into the details of life, and enslaving the soul itself.”

John Stuart Mill’s view on liberty, which was influenced by Joseph Priestley and Josiah Warren, is that the individual ought be free to do as he wishes unless he harms others. Individuals are rational enough to make decisions about their good being and choose any religion they want to. Government should interfere when it is for the protection of society. An influential advocate of freedom of speech, Mill objected to censorship.

In 1850, Mill sent an anonymous letter (which came to be known under the title "The Negro Question"), in rebuttal to Thomas Carlyle's anonymous letter to Fraser's Magazine for Town and Country. Carlyle had defended slavery on grounds of genetic inferiority and claimed that the West Indies development was due to British ingenuity alone and dismissed any notion that there was a debtedness to imported slaves for building the economy there. Mill's rebuttal and references to the ongoing debate in the U.S. at the time regarding slavery were emphatic and eloquent. Mill is also famous for being one of the earliest and strongest supporters of ever greater rights for women. His book The Subjection of Women is one of the earliest written on this subject by a male author. He felt that the 'oppression' of women was one of the few remaining relics from ancient times, a set of prejudices that severely impeded the progress of humanity.

The canonical statement of Mill's utilitarianism can be found in Utilitarianism. This philosophy has a long tradition, although Mill's account is primarily influenced by Jeremy Bentham and Mill's father James Mill. Mill's famous formulation of utilitarianism is known as the "greatest-happiness principle". It holds that one must always act so as to produce the greatest happiness for the greatest number of people, within reason. Mill's major contribution to utilitarianism is his argument for the qualitative separation of pleasures. Bentham treats all forms of happiness as equal, whereas Mill argues that intellectual and moral pleasures are superior to more physical forms of pleasure. Mill distinguishes between happiness and contentment, claiming that the former is of higher value than the latter, a belief wittily encapsulated in the statement that "[i]t is better to be a human being dissatisfied than a pig satisfied; better to be Socrates dissatisfied than a fool satisfied. And if the fool, or the pig, are of a different opinion, it is because they only know their own side of the question."

Mill defines the difference between higher and lower forms of happiness with the principle that those who have experienced both tend to prefer one over the other. This is, perhaps, in direct contrast with Bentham's statement that "Pushpin is as good as Poetry", that, if a simple child's game like hopscotch causes more pleasure to more people than a night at the opera house, it is more imperative upon a society to devote more resources to propagating hopscotch than running opera houses. Mill's argument is that the "simple pleasures" tend to be preferred by people who have no experience with high art, and are therefore not in a proper position to judge. Mill supported legislation that would have granted extra voting power to university graduates on the grounds that they were in a better position to judge what would be best for society. It should be noted that, in this example, Mill did not intend to devalue uneducated people and would certainly have advocated sending the poor but talented to universities: he believed that education, and not the intrinsic nature of the educated, qualified them to have more influence in government.

Mill also dealt with one of the prime problems associated with utilitarianism, that of schadenfreude. Detractors of utilitarianism argued, among other things, that, if enough people hated another person sufficiently that simply reducing the happiness of the object of their hatred would cause them pleasure, it would be incumbent upon a utilitarian society to aid them in harming the individual. Mill argued that, in order to have such an attitude of malice, each citizen would have to value his own pleasure over that of another. Society, therefore, is in no way obligated to indulge him; on the contrary, it is fully permitted to suppress his actions.

Mill's early economic philosophy was one of free markets. However, he accepted interventions in the economy, such as a tax on alcohol, if there were sufficient utilitarian grounds. He also accepted the principle of legislative intervention for the purpose of animal welfare. Mill originally believed that "equality of taxation" meant "equality of sacrifice" and that progressive taxation penalized those who worked harder and saved more and was therefore "a mild form of robbery". Given a tax break to the rich, Mill agreed that inheritance should be taxed. A utilitarian society would agree that everyone should be equal one way or another. Therefore receiving inheritance would put one ahead of society unless taxed on the inheritance. Those who donate should consider and choose carefully where their monies go. Some charities are more deserving than others. Considering public charities boards such as a government will disperse the monies equally. However a private charity board like a church would disperse the monies fairly to those whom are in more need than others. Later he altered his views toward a more socialist bent, adding chapters to his Principles of Political Economy in defense of a socialist outlook, and defending some socialist causes. Within this revised work he also made the radical proposal that the whole wage system be abolished in favour of a co-operative wage system. Nonetheless, some of his views on the idea of flat taxation remained, albeit in a slightly toned down form. Mill promoted economic democracy in the capitalist economy whereby labourers would elect members of management. Mill believed that this was necessary to end what he deemed to be dictatorial management of capitalist firms and to establish liberty and equality in the capitalist economy. Mill's promotion of the right of labourers to elect management has been seen as support for economic corporatism.

Mill demonstrated an early insight into the value of the natural world - in particular in Book IV, chapter VI of "Principles of Political Economy": "Of the Stationary State" in which Mill recognised wealth beyond the material, and argued that the logical conclusion of unlimited growth was destruction of the environment and a reduced quality of life. He concluded that a stationary state, rather than economic growth, was ideal:

If the earth must lose that great portion of its pleasantness which it owes to things that the unlimited increase of wealth and population would extirpate from it, for the mere purpose of enabling it to support a larger, but not a better or a happier population, I sincerely hope, for the sake of posterity, that they will be content to be stationary, long before necessity compel them to it.